1. Field of the Invention
The present invention relates to a sputtered mask defined with highly selective side wall chemical etching and, more particularly, to such a mask wherein chemical etching of the side wall of the mask is many orders of magnitude more than chemical etching of a flat surface of the mask.
2. Description of the Related Art
A write head comprises first and second pole pieces that have first and second pole tips terminating at an air bearing surface and ends recessed from the ABS (air bearing surface) that are connected at a back gap. An insulation stack, which comprises a plurality of insulation layers, is sandwiched between the first and second pole pieces, and a coil layer is embedded in the insulation stack. A processing circuit is connected to the coil layer for conducting current (write signals) through the coil layer which, in turn, generates corresponding magnetic fields on the first and second pole pieces. When they meet the ABS, the first and second pole pieces form first and second pole tips. A non-magnetic insulative gap layer is sandwiched between the first and second pole tips so that the magnetic fields fringe across the first and second pole tips at the ABS. In a magnetic disk drive, a magnetic disk is rotated adjacent to and a short distance from the ABS so that the fringing magnetic fields magnetize the disk along circular tracks. The written circular tracks then contain information in the form of magnetized regions that can be detected by a read head.
A write head and read head may be combined to form a merged magneto-resistive (MR) head. The read head includes an MR sensor sandwiched between first and second insulative gap layers that are, in turn, sandwiched between first and second shield layers. In a merged MR head, a single layer may function as the second shield and the first pole. The MR sensor detects magnetic fields from the circular tracks of the rotating disk when its resistance changes in response to the strength and polarity of the fields. A sense current is conducted through the MR sensor, which results in voltage changes that are received by the processing circuitry as readback signals.
The second pole tip trails the first pole tip with respect to the rotating disk and is therefore the last of the two pole tips to impress information field signals on the circular tracks. The second pole tip is bounded by a base, which sits directly on the gap layer, a top, and first and second side walls. The first and second side walls intersect the base at first and second corners. There is a strong-felt need to provide the second pole tip with a narrow track width--the lateral distance between the first and second side walls at the base. A narrow track width increases the number of tracks that can be recorded per inch (TPI) on the magnetic disk. Narrow track width thus implies a smaller magnetic disk drive for a given number of recorded bits.
It is important that the side walls of the second pole tip, especially at the base, be well formed in a linear configuration so that magnetic field fringing at the pole tips is confined substantially to the track width of the second pole tip. Side writing occurs when the magnetic fields fringe from irregular side walls of the second pole tip to the first pole tip, laterally beyond the track width of the second pole tip. As is known, side writing may cause overwriting of the circular tracks, which reduces the track density of the magnetic disk. Ideally, the second pole tip should write well-defined narrow tracks which, in turn, are read by a read head that reads slightly more narrowly than the written track. This obviates the need for guard bands between tracks.
The second pole, along with its second pole tip, is frame-plated on top of the gap layer. After depositing a seedlayer on the gap layer, a photoresist layer is spun on the seedlayer, imaged with light, and developed to provide an opening surrounded by a resist wall for plating the second pole piece and second pole tip. To produce a second pole tip with a narrow track width, the photoresist layer has to be correspondingly thin. This relationship, referred to as the "aspect ratio", is the ratio of the thickness of the photoresist in the pole tip region to the track width of the second pole tip. Preferably, the aspect ratio should be on the order of three. In other words, for a track width of 1 .mu.m, the thickness of the photoresist in the pole tip region should be about 3 .mu.m. If the photoresist is thicker than this, the side walls of the second pole tip, especially at the base, will not be well formed due to scattering of light as it penetrates the photoresist during the imaging step.
Once the second pole tip is well formed, it is desirable to notch the first pole tip opposite the first and second corners at the base of the second pole tip. Here, the gap layer is bounded by a base that rests on the first pole piece, a top that engages the base of the second pole tip, and first and second side walls that intersect the first and second side walls at first and second corners, respectively. Notching of the first pole piece occurs immediately adjacent each of the first and second corners of the gap layer. Notching provides the first pole piece with a track width that substantially matches the track width of the second pole piece so as to minimize side writing.
A prior art process for notching the first pole piece entails ion beam milling the gap layer and the first pole piece, employing the second pole tip as a mask. The gap layer is typically alumina, while the first and second pole pieces and pole tips are typically Permalloy (NiFe). Alumina mills more slowly than the Permalloy; thus the top of second pole tip and a top surface of the first pole piece are milled more quickly than the gap layer. Further, during ion milling, there is much redeposition (redep) of alumina on surfaces of the workpiece. In order to minimize redep, the milling ion beam is typically directed at an angle to a normal to the layers, which performs milling and cleanup simultaneously. The gap layer in the field remote from the first and second corners of the second pole piece is the first to be milled due to a shadowing effect at the first and second corners when the ion beam is angled. In this case, the ion beam may overmill the first pole piece before the gap layer is removed adjacent the first and second corners of the second pole tip in the region where the notching is to take place. After the gap layer is removed above the sites where the notching is to take place, ion milling continues in order to notch the first pole piece at the sites adjacent the first and second corners of the gap layer. Again, with an angled ion beam overmilling of the first pole piece takes place in the field beyond the progressively formed notches, thereby forming surfaces of the first pole piece that slope downwardly from the first and second corners of the gap layer. As is known, such overmilling of the first pole piece can easily expose leads to the MR sensor and the second gap layer of the read head, rendering the head inoperative.
Even if overmilling of the first pole piece can be controlled, a potentially more serious problem may occur, namely overmilling the top of the second pole tip when the unwanted portions of the gap layer are milled and notches are formed. In order to compensate for this overmilling, the aspect ratio is increased so that a top portion of the top of the second pole tip can be sacrificed during the milling steps. As already stated, when the aspect ratio is increased, definition of the second pole tip is degraded, resulting in track overwriting.
In order to minimize overmilling of the first pole piece, another process employs a wet etchant for removing the gap layer, except for a desired portion between the first and second pole tips. After the unwanted portions of the gap layer are removed, the first pole piece is ion milled, employing the second pole tip as a mask. The only overmilling of the first pole piece is due to the ion milling of the notches at the first and second corners of the gap layer. This process also eliminates significant redep of the alumina. However, the etching of this process undercuts the gap layer under the base of the second pole tip, which is a critical area for the transfer of field signals. The undercut regions provide spaces which can be filled with Permalloy redeposited during subsequent ion milling of the first pole piece or redep of other foreign material upon subsequent milling and clean up steps.
Still another process proposes plating the top and first and second side walls of the second pole tip with a protective metal layer before etching the unwanted portions of the gap layer. When the etching reaches the inside thickness of each protective metal layer, the process is stopped so that the gap layer is not undercut beneath the base of the second pole tip. Retention of the protective metal layer in the head is an option because of the difficulty of removing it. Disadvantages of this process are the difficulty of the plating step and the potential of the protective metal layer interfering with the magnetics of the second pole tip.